1. Field of the Invention
The present invention relates to improved nonwoven fabrics and melt spinning apparatus and processes for producing such nonwoven fabrics. In particular, the present invention involves the extrusion of ribbon-shaped fibers and the formation of nonwoven fabrics made from synthetic ribbon-shaped fibers having superior coverage, filtration, strength and elongation properties.
2. Description of the Related Art
The most common synthetic textile fibers used in nonwoven fabrics are made from materials such as nylon, polyester or polypropylene polymers. All of these polymers are melt spinnable. Some nonwoven fabrics made from carded or air-laid 25 webs comprise rayon or acrylic fibers. Many of the nonwoven fabrics made from melt-spinnable polymers are produced using a spunbond process. The term xe2x80x9cspunbondxe2x80x9d refers to a process of forming a nonwoven fabric or web from thin fibers or filaments produced by extruding molten polymers from orifices of a spinneret. The filaments are drawn as they cool (e.g., by an aspirator, positioned below the spinneret, which longitudinally stretches and transversely attenuates the fibers) and are randomly laid on a forming surface, such that the filaments form a nonwoven web. The web is subsequently bonded using one of several known techniques to form the nonwoven fabric. Carded or air-laid webs can also be formed from these polymers.
Fibers having a round (i.e., circular) transverse cross-sectional shape, as shown in FIG. 1, are the most common and least expensive melt spun fibers used to form nonwoven fabrics. Such fibers have a number of limitations, however. For example, fibers having a circular transverse cross-sectional shape tend to be relatively stiff and do not bend as readily as fibers of other cross-sectional shapes; consequently, these round fibers tend to produce fabrics having a texture that is less soft.
For a given fabric basis weight, the shape and stiffness of round fibers produce fabrics having limited surface area coverage, i.e., a significant amount of open area is present between the fibers of the fabric relative to fibers having other cross-sectional shapes. This limited coverage results in a limited ability of the fabric to serve as a filter or barrier material, since gasses, fluids and particulate matter can pass through the gaps or holes between fibers with relative ease.
Further, round fibers inherently have a limited fiber surface area, which has a number of implications for the spunbond process for forming nonwoven fabric as well as for the properties of the fabric itself. Specifically, round fibers extruded in a molten state quench first at the fiber surface and more slowly in the center of the fiber. Significant molecular orientation cannot take place while the polymer is still molten; hence, only the fiber surface is well oriented. This makes the fiber less strong than if it were equally well oriented throughout its cross-section.
The use of round fibers also limits the efficiency of the aspirator. The aspirator or other fiber-drawing mechanism, is designed to longitudinally stretch and transversely attenuate the fibers as they travel substantially vertically downward from the spinneret. This drawing of the fibers is achieved by applying a downward air drag on the fibers, which air drag is produced by air pressure creating a generally-downward, high-velocity air flow. Because of the limited surface area of round fibers, a limited downward air drag on the fibers is induce by the downward air flow, thereby limiting the amount of fiber stretching and attenuation for a given aspirator air pressure and as well as limiting the energy efficiency of the aspirator.
With limited surface area, circular fibers quench relatively slowly and remain in a molten or soft state for a relatively long time after extrusion; consequently, the aspirator used to draw the fibers must be located a significant distance away from the spinneret from which the fibers are extruded to allow sufficient quenching time in order to prevent the fibers from sticking to components of the aspirator. The requirement for this distance between the spinneret and the aspirator causes significant unwanted air drag associated with length of the fiber between the spinneret and aspirator (to be distinguished from the desired downward air drag produced by the aspirator), and reduces the efficiency of the aspirator by requiring more aspirator drawing force (i.e., air pressure) to overcome this pre-aspirator air drag (with less of the drawing force contributing to stretching and attenuating the fibers).
The relatively low surface area of round fibers also limits the usefulness of fabrics made from such fibers in filtration and barrier material applications. Specifically, the round fibers present a limited surface area for collecting or blocking dirt, gasses or fluids. Further, it is more difficult to apply finish oil or other topical treatments to fabrics formed from round fibers.
Fibers having other transverse cross-sectional shapes, such as a delta shape (FIG. 2) or a Y shape tend to give a slightly greater fiber stiffness relative to fibers having a round transverse cross section, and may also add sparkle to the fiber appearance. Hollow fibers can conserve polymer and reflect light in a desirable manner. FIG. 4 illustrates a conventional hollow fiber having a circular transverse cross-sectional shape and a single, concentric longitudinal cavity. A plural cavity hollow fiber is shown in FIG. 5 in which four circular longitudinal cavities arranged transversely in a square pattern extend through a fiber having a substantially square transverse cross-sectional shape. Thus, fibers whose transverse cross sections are other than round can provide certain advantages. However, none of these fibers overcomes all of the aforementioned limitations of round fibers.
While some use has been made of fibers with flat or ribbon-shaped cross sections, there have been no known attempts to form nonwoven fabrics from fibers extruded with a ribbon-shaped cross section. Nor has there been any significant investigation into the possible advantages of using such extruded ribbon-shaped fibers in nonwoven fabrics.
U.S. Pat. No. 5,498,468 to Blaney, the disclosure of which is incorporated herein by reference in its entirety, discloses a process of extruding sheath-core conjugate filaments having a substantially circular transverse cross-sectional shape, and applying a flattening force to the filaments with a calendar roll arrangement to flatten the circular filaments into ribbon-like filaments. The method requires that the core polymer have a lower softening point than the sheath polymer such that, when the filaments are heated to the softening point of the core polymer, the flattening force causes the filaments to deform in accordance with deformation of the core without causing the sheath to soften, thereby preventing adjacent fibers from fusing in the flattening process. While fabrics formed from the flattened fibers purportedly exhibit superior coverage properties (i.e., reduced open area in the fabric), the fibers are extruded with circular cross-sectional shapes; consequently, any potential advantages of extruding ribbon-shaped fibers are not suggested by and cannot be realized in this system. Moreover, the process is limited to sheath-core conjugate fibers wherein the core component has a lower softening point than the sheath component; thus, the process does not have general applicability to fibers other than those with this specific conjugate fiber configuration.
U.S. Pat. No. 5,593,768 to Gessner, the disclosure of which is incorporated herein by reference in its entirety, discloses a process for forming a nonwoven fabric laminate from a thermally bonded multiconstituent fiber nonwoven web. Specifically, the fibers of the web are formed of two highly intermixed polymer components, one of which has a lower softening point which facilitates bonding of the fibers in the web. In order to enhance bonding, it is desirable to maximize the amount of the low softening point component that is at the surface of the fibers. As explained in the disclosure, this can be achieved by increasing the surface-to-volume ratio of the fiber, which is an attribute of ribbon-shaped fibers. However, while this patent appears to suggest an advantage of ribbon-shaped fibers in the narrow context of thermal bonding of plural intermixed component fibers, the disclosure contains no suggestion to extrude ribbon-shaped fibers, and, if anything, the examples provided in the disclosure suggest that a ribbon-shape is achieved by flattening round fibers during the thermal bonding process.
It is an object of the present invention to produce nonwoven fabric having enhanced coverage for a given basis weight.
It is a further object of the present invention to produce nonwoven fabric with increased surface area for a given basis weight.
It is a still further object of the present invention to produce nonwoven fabric having superior fluid and/or particulate filtration and barrier properties.
It is another object of the present invention to produce soft, flexible nonwoven fabric.
It is yet another object of the present invention to form a fiber web that can more readily be bonded to form nonwoven fabric.
It is still another object of the present invention to produce nonwoven fabric to which a topical treatment can more readily be applied.
A further object of the present invention is to increase the energy efficiency of a spunbond process by reducing the energy required to draw extruded fibers, reducing the suction force required to deposit fiber on a web-forming surface, and/or reducing the energy required to bond the fiber web.
A still further object of the present invention is to achieve higher fiber velocities while drawing extruded fibers without increasing aspirator air pressure in order to achieve lower denier fibers without an attendant increase in energy cost.
Another object of the present invention is to produce stronger nonwoven fabric.
Yet another object of the present invention is to produce nonwoven fabric having superior elongation properties.
Still another object of the present invention is to more rapidly quench extruded polymeric fibers, enabling reductions in capital equipment and fiber production costs.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
According to the present invention, nonwoven fabric is formed from a spunbond process by extruding generally ribbon-shaped fibers through slot-shaped orifices of a spinneret. The ribbon-shaped fibers are rapidly quenched after extrusion to achieve a substantially uniform molecular orientation throughout a transverse cross section of the fibers, yielding stronger fibers. The rapid quenching results largely from the relatively high aspect ratio (thinness) of the fibers and the relatively large surface area of the fibers, which permits the fibers to quickly cool throughout the transverse cross section. The ribbon-shaped fibers are drawn longitudinally by exerting a generally downward force produced by an air stream that longitudinally stretches and transversely attenuates the ribbon-shaped fibers in such a manner that the transverse cross-sectional shape of the ribbon-shaped fibers enhances the interaction between the air stream and the ribbon-shaped fibers to maximize the downward force. The attenuated ribbon-shaped fibers are deposited onto a web forming surface, such as a moving wire screen belt to form a web. The web is then bonded to form the nonwoven fabric.
The formation of nonwoven fabric from ribbon-shaped fibers provides a number of unexpected benefits. In particular, fabric of a given basis weight has more coverage when formed from ribbon-shaped fibers rather than from round fibers or fibers of other cross-sectional shapes. Nonwoven fabric formed from ribbon-shaped fibers has enhanced filtration properties, because the fabric collects more dirt and/or pass less fluid (gas or liquid) due to the greater fiber surface area for a given basis weight. Further, nonwoven fabric formed of ribbon-shaped fibers is softer than nonwoven fabric formed of fibers of other transverse cross-sectional shapes, since each ribbon-shaped fiber can bend more easily, and the ribbon-shaped fibers can be bonded more easily than round fibers, since the flat surface of adjacent ribbon-shaped fibers in contact with each other form a greater contact area between the fibers.
Further, the greater surface area of each ribbon-shaped fiber allows the downwardly directed air in the aspiration to xe2x80x9cgripxe2x80x9d the fiber better due to increased downward drag on the fibers, hence achieving a fiber velocity closer to the aspirator""s downward air velocity. This increased fiber velocity results in fibers of desirably lower denier at a given air pressure and air consumption. The rapid quenching permits the aspirator to be located closer to the spinneret without the danger of the fibers sticking to the metal aspirator parts. This also reduces upstream air drag on the fibers, allowing the fiber tension generated by the aspirator to carry up to the spinneret with less loss due to the upstream air drag, with more of the aspirator drawing force contributing to attenuation of the ribbon-shaped fibers. The higher fiber tension at the spinneret results in still lower denier and still greater orientation.
Since the ribbon-shaped fibers of the present invention bend very easily in one direction, they tend to bend in that direction and lay down in smaller coils than round fibers and are more easily carried by the fan-driven laydown air into open areas amongst the fibers already laid down, thereby tending to fill in holes in the web and to make a more uniform looking fabric. The ribbon-shaped fibers tend to lay down preferentially more in the machine direction than in the direction transverse thereto. As a result, the nonwoven fabric has a greater strength and stretchability in the machine direction than a comparable fabric formed from round fibers.
The present invention is not limited to nonwoven fabrics formed from spunbond process and encompasses process for forming fabric from ribbon-shaped fibers that do not require bonding of the fibers (e.g., spun-laid or air carding processes). Further, the present invention can be applied in melt blown systems. The benefits of using ribbon-shaped fibers are not limited to systems that form webs from continuous filaments, and the present invention encompasses processes for forming nonwoven fabrics from ribbon-shaped staple fibers.
Further, formation of ribbon-shaped fibers in accordance with the present invention can be performed in conjunction with other extrusion and fabric or material formation techniques. For example, both ribbon-shaped and non-ribbon shaped-fibers can be extruded from a single spinneret to create a web having a mixture of different types of fiber shapes. Further, a web formed from ribbon-shaped fibers can be coupled to (e.g., bonded to) non-ribbon webs or laminates in, for example, a multi-layered product.
The nonwoven fabric formed by the process of the present invention is useful in any product where properties such as softness, strength, stretchability, filtration or fluid barrier properties, and high coverage at a low fabric weight are desirable or advantageous, including, but not limited to: disposable absorbent articles; medical barrier fabrics; filtration media; and clothing liners.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.